The present invention relates to a circular pole piece included in a magnetic circuit for magnetic resonance imaging (MRI) and an MRI system. More particularly, the present invention relates to a circular pole piece capable of minimizing a residual magnetic induction and an MRI system employing the circular pole piece.
In the past, a circular pole piece included in a magnetic circuit for magnetic resonance imaging (MRI) has been shaped substantially circularly as a whole by arranging laminate blocks each of which has square or rectangular silicon steel sheet tiles or heteromorphic soft magnetic material tiles, each of which is used to fill a gap among adjoining square or rectangular tiles, layered (see, for example, FIG. 1 in Patent Document 1). Otherwise, the circular pole piece has been shaped substantially circularly as a whole by arranging laminate blocks, each of which has trapezoidal or annularly sectoral soft magnetic material tiles, layered in the form of multiple concentric rings (see, for example, FIG. 21 in the Patent Document 1).
The soft magnetic material tiles are made of, for example, a silicon steel sheet and exhibit a hysteresis. Therefore, after a magnetic field gradient is applied to the circular pole piece, a residual magnetic induction occurs in the circular pole piece. The magnetic induction varies depending on a change in the magnetic field gradient. However, when the residual magnetic induction in the circular pole piece varies, it affects an image.
In the past, efforts have been made to devise a pulse sequence that is effective in suppressing the adverse effect (refer to, for example, Patent Document 2).
[Patent Document 1]
Japanese Unexamined Patent Application Publication No. 2000-200716
[Patent Document 2]
Japanese Unexamined Patent Application Publication No. 2000-185023
In the past, the adverse effect of the residual magnetic induction in a circular pole piece has been suppressed by devising a pulse sequence. However, this imposes a load on an MRI system.